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From Aljoscha Krettek <aljos...@apache.org>
Subject Re: [DISCUSS] Change Streaming Operators to be Push-Only
Date Tue, 05 May 2015 13:21:17 GMT
I'm using the term punctuation and watermark interchangeably here
because for practical purposes they do the same thing. I'm not sure
what you meant with your comment about those.

For the Operator interface I'm thinking about something like this:

abstract class OneInputStreamOperator<IN, OUT, F extends Function>  {
    public processElement(IN element);
    public processBarrier(...);
    public processPunctuation/lowWatermark(...):
}

The operator also has access to the TaskContext and ExecutionConfig
and Serializers. The operator would emit values using an emit() method
or the Collector interface, not sure about that yet.

On Tue, May 5, 2015 at 3:12 PM, Gyula Fóra <gyfora@apache.org> wrote:
> I think this a good idea in general. I would try to minimize the methods we
> include and make the ones that we keep very concrete. For instance i would
> not have the receive barrier method as that is handled on a totally
> different level already. And instead of punctuation I would directly add a
> method to work on watermarks.
>
> On Tuesday, May 5, 2015, Aljoscha Krettek <aljoscha@apache.org> wrote:
>
>> What do you mean by "losing iterations"?
>>
>> For the pros and cons:
>>
>> Cons: I can't think of any, since most of the operators are chainable
>> already and already behave like a collector.
>>
>> Pros:
>>  - Unified model for operators, chainable operators don't have to
>> worry about input iterators and the collect interface.
>>  - Enables features that we want in the future, such as barriers and
>> punctuations because they don't work with the
>>    simple Collector interface.
>>  - The while-loop is moved outside of the operators, now the Task (the
>> thing that runs Operators) can control the flow of data better and
>> deal with
>>    stuff like barriers and punctuations. If we want to keep the
>> main-loop inside each operator, then they all have to manage input
>> readers and inline events manually.
>>
>> On Tue, May 5, 2015 at 2:41 PM, Kostas Tzoumas <ktzoumas@apache.org
>> <javascript:;>> wrote:
>> > Can you give us a rough idea of the pros and cons? Do we lose some
>> > functionality by getting rid of iterations?
>> >
>> > Kostas
>> >
>> > On Tue, May 5, 2015 at 1:37 PM, Aljoscha Krettek <aljoscha@apache.org
>> <javascript:;>>
>> > wrote:
>> >
>> >> Hi Folks,
>> >> while working on introducing source-assigned timestamps into streaming
>> >> (https://issues.apache.org/jira/browse/FLINK-1967) I thought about how
>> >> the punctuations (low watermarks) can be pushed through the system.
>> >> The problem is, that operators can have two ways of getting input: 1.
>> >> They read directly from input iterators, and 2. They act as a
>> >> Collector and get elements via collect() from the previous operator in
>> >> a chain.
>> >>
>> >> This makes it hard to push things through a chain that are not
>> >> elements, such as barriers and/or punctuations.
>> >>
>> >> I propose to change all streaming operators to be push based, with a
>> >> slightly improved interface: In addition to collect(), which I would
>> >> call receiveElement() I would add receivePunctuation() and
>> >> receiveBarrier(). The first operator in the chain would also get data
>> >> from the outside invokable that reads from the input iterator and
>> >> calls receiveElement() for the first operator in a chain.
>> >>
>> >> What do you think? I would of course be willing to implement this
>> myself.
>> >>
>> >> Cheers,
>> >> Aljoscha
>> >>
>>

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